1. Field of Invention
The present invention relates generally to the field of coalbed methane production, and more specifically to methods for application of fluids or materials into subsurface coal seams that release free oxygen to create a rapid oxidation reaction within the coal seam in order to stimulate natural gas production from the coal seam.
2. Related Art
Commercial natural gas production from subsurface coal seams has now entered its third decade. Subsurface coal seams may contain a large amount of natural gas or methane (commonly referred to as coalbed methane, or CBM) that is adsorbed onto the surface of the coal. This gas is released from the coal and may be produced when the pressure is significantly reduced in the coal seam. However in most cases the depressurization (and thus the gas production) is curtailed by either low permeability in the coal, or because of damage to the coal during the drilling or completion process.
To date there are two methods of stimulation or bypassing damaged coals to increase the amount of gas production: a) cavitation; or b) hydraulic fracturing. Cavitation is a method of removing coal through repeated injections of fluids and aggressive flowbacks to shear off and produce coal up a wellbore, thus enlarging the wellbore by creating a cavity. Unfortunately this method has been successful only in a very limited amount of coal seams containing coal having specific friable properties.
The other method, hydraulic fracturing, is much the same method that has been applied in conventional oil and gas formations for years. This involves inducing fractures in the coal seams by pumping fluids into the formation at high pressures and at high rates. Unfortunately, due to the soft nature of the coals and to the presence of natural fractures (called cleats), these induced hydraulic fractures have not been very efficient and far underperform similar applications in conventional oil and gas formations. Proppant has been added to the fracturing fluid to enhance the fracture conductivity after the hydraulic pressure is bled off; however premature proppant bridging has been a problem in coal seam fracturing. Often, high viscosity fluids were required to successfully place these proppant treatments. However, these high viscosity fluids often cause secondary damage to the coal cleats adjacent to the fracture, which could greatly temper the stimulation effects of the fracture treatment.
Coal is a subterranean formation composed largely of carbon compounds, for example having a typical composition of about (85% C, 5% H, 5% (O,N,S) 5% M), in which C refers to total carbon content (fixed plus volatile matter); H refers to total hydrogen content; O,N,S refers to the total of oxygen, plus nitrogen, plus sulphur; and M refers to the total content of inert matter. Coal and carbonates (limestones and dolomites) are often sources of oil and gas production and are often naturally fractured, which enhances their potential productivity. Coal, limestones and dolomites may have limited oil and gas productivity due to low permeability or to damage during drilling and completion. However, the carbonates may be stimulated readily or their damage may be bypassed because the rock may be dissolved readily with cost effective acid, such as hydrochloric acid. The limestone/HCl dissolution reaction is:2HCl+CaCO3<-->CaCl2+H2O+CO2 The dolomite/HCl dissolution reaction is:4HCl+CaMg(CO3)2<-->CaCl2+MgCl2+2H2O+2CO2 These formations can be stimulated by enlarging the wellbore and removing or bypassing damage, or hydraulic fractures can be enhanced by fracturing with an acidic fluid which will remove rock along the fracture face and enhance the permeability of the fracture after hydraulic pressure is removed.
Several efforts have been made to use oxidizers for increasing CBM production, however none of these describes or suggests using combustion enhanced by providing an oxidizer for rock removal in stimulation of CBM production. There is a continuing and as yet unmet need for increasing CBM production.